This invention relates to communication privacy and more particularly, to scramblers that manipulate speech signals.
Speech scramblers are increasingly used for commercial as well as government communication. Digital scramblers have been developed in which a speech signal is encrypted and transmitted digitally. Conventional analog radio and telephone channels, however, cannot support the digital transmission rates required for high quality voice. Analog scramblers, those in which the final output of the scrambling process is an analog signal, therefore continue to be important.
In U.S. Pat. No. 4,278,840, issued to B. Morgan et al. on July 14, 1981, the speech spectrum is shifted within a predetermined band. The bandshifted signal is then divided into time segments. The time segments are permuted according to a pseudo-random process and transmitted. The Morgan disclosure illustrates a class of analog scramblers in which time and frequency operations occur in the tandem, that is, with one operation following the other. Although such tandem processes may have low residual intelligibility, they also may have, however, comparatively low cryptanalytical strength.
It is thus an object of the invention to provide an improved analog scrambler having enhanced cryptanalytical strength and low residual intelligibility.